The present invention relates to a gas diffusion layer for a gas diffusion electrode. More specifically, the invention relates to a gas diffusion layer used for a gas diffusion electrode for a fuel gas. The present invention relates to a fuel cell including a gas diffusion electrode using the aforesaid gas diffusion layer.
Conventional polymer electrolyte fuel cells employing a cation (hydrogen ion) conductive polymer electrolyte generate electricity and heat by electrochemically reacting a fuel gas containing hydrogen and an oxidant gas containing oxygen such as air.
FIG. 5 is a schematic cross sectional view illustrating a basic structure of a unit cell designed to be mounted in a conventional polymer electrolyte fuel cell. FIG. 6 is a schematic cross sectional view illustrating a basic structure of a membrane-electrode assembly designed to be mounted in the unit cell 100 shown in FIG. 5. As shown in FIG. 6, in a membrane-electrode assembly 101, on each surface of a polymer electrolyte membrane 111 capable of selectively transporting hydrogen ions is formed a catalyst layer 112 composed of a hydrogen ion conductive polymer electrolyte and a catalyst body obtained by allowing carbon powders to carry an electrode catalyst (e.g. platinum metal catalyst).
As the polymer electrolyte membrane 111, polymer electrolyte membranes made of perfluorocarbonsulfonic acid such as Nafion (trade name) available from E.I. Du Pont de Nemours & Co. Inc., USA are now widely used.
On the outer surface of each catalyst layer 112 is formed a gas diffusion layer 113 having gas permeability and electron conductivity by using carbon paper, which has been subjected to water repellent treatment, for example. The combination of the catalyst layer 112 and the gas diffusion layer 113 constitutes a gas diffusion electrode 114 (anode or cathode).
A conventional unit cell 100 is composed of a membrane-electrode assembly 101, gaskets 115 and a pair of separators 116. The gaskets 115 are arranged on the outer periphery of the electrodes with the polymer electrolyte membrane 111 sandwiched therebetween so as to prevent the supplied fuel gas and the supplied oxidant gas from leaking out and to prevent them from mixing with each other. The gaskets 115 are usually integrated in advance with the electrodes and the polymer electrolyte membrane 111. In some cases, the combination of the electrodes and the polymer electrolyte membrane 111 and gaskets 115 is called “membrane-electrode assembly”.
On the outer surfaces of the membrane-electrode assembly 101 are placed a pair of separators 116 for mechanically fixing the membrane-electrode assembly 101. On the surface of the separator 116 in contact with the membrane-electrode assembly 101 is formed gas channels 117 for supplying a reaction gas (fuel gas or oxidant gas) to the gas diffusion electrode 114 and removing a gas containing an electrode reaction product and unreacted reaction gas from the reaction site to the outside of the electrodes.
Although the gas channels 117 may be formed independently of the separator 116, they are usually formed by providing grooves on the surface of the separator as shown in FIG. 5.
A single unit cell 100 constructed by fixing the membrane-electrode assembly 101 with a pair of separators 116 can produce an electromotive force of about 0.7 to 0.8 V at a practical current density of several tens to several hundreds mA/cm2 when a fuel gas is supplied to the gas channel 117 of one of the separators 116 and an oxidant gas is supplied to the gas channel 117 of the other of the separators 116.
Polymer electrolyte fuel cells, however, are usually required to produce a voltage of several to several hundreds volts when used as power sources. For this reason, in practice, the required number of unit cells are connected in series to give a stack for use.
In order to supply a reaction gas to the gas channel 117, there is required a manifold in which a pipe for supplying the reaction gas is branched into a corresponding number of separators 116 and the branched pipes are directly connected to the gas channels on the separators 116. Particularly, a manifold in which external pipes for supplying the reaction gas are directly connected to the separators 116 is called “external manifold”.
On the other hand, there is another type of manifold called “internal manifold”, which has a simpler structure. An internal manifold is composed of apertures formed in the separators 116 having gas channels 117 formed thereon. The inlet and outlet of the gas channel 117 are connected with the apertures. The reaction gas can be supplied to the gas channel 117 directly from the aperture.
The gas diffusion layer (GDL) 113 is also called as “GDM” (gas diffusion media) or “backing layer”. It serves to supply chemical substances as the energy source for power generation to the catalyst layer 112 uniformly without any loss during transportation, and to remove the generated electric energy to an external circuit. For this reason, the gas diffusion layer 113 is required to have high gas permeability for reaction gas such as fuel gas or oxidant gas, high water drainage capability and high electron conductivity.
In order to impart the gas permeability, the gas diffusion layer 113 has a porous structure, whereby a reaction gas can be supplied and diffused uniformly to the catalyst in the catalyst layer 112 from the gas channel 117 arranged such that it is in contact with the gas diffusion layer 113.
In order to impart water drainage capability to the gas diffusion layer 113 so as to facilitate the removal of water produced by the reaction in the catalyst layer 112, a water repellent polymer as typified by fluorocarbon resin or the like is dispersed in the pores of the gas diffusion layer 113. Imparting water repellency to the gas diffusion layer 113 like this prevents the clogging of water (flooding) in the gas diffusion layer 113.
Further, the gas diffusion layer 113 is also required to have a function to conduct electrons necessary for the reaction and generated electrons. In order to acquire such electron conductivity, the gas diffusion layer 113 is formed using an electron conductive material such as carbon fiber, metal fiber or carbon fine powders.
A conventional gas diffusion layer 113 like the one as described above is typically produced by first coating a porous electron conductive material such as carbon paper or carbon cloth serving as the substrate for the gas diffusion layer 113 with a water repellent resin such as fluorocarbon resin, and then forming a water repellent conductive layer (water repellent carbon layer)(not shown in the drawings) containing carbon and a water repellent polymer on one surface of the coated substrate, that is, the surface to be in contact with the catalyst layer 112.
Although the gas diffusion layer 113 is usually produced in the above-described manner, in order to cope with the recent trend of providing higher output, prolonging service life and reducing cost, various attempts have been made on the development of production methods therefor and the shape of the finished products.
For example, in an attempt to enhance water drainage capability by the gas diffusion layer 113, Japanese Laid-Open Patent Publication No. 2001-283869 proposes to form a water repellent carbon layer having asperities, on the surface of the gas diffusion layer 113 to be in contact with the catalyst layer 112, so as to increase the evaporation area where the water produced by power generation can evaporate.
In Japanese Patent Publication No. 2831061, in an attempt to achieve a high output and cost reduction by increasing the contact area between the catalyst in the catalyst layer 112 and the electrolyte in the polymer electrolyte membrane 111 to significantly improve the utilization of the catalyst, there is proposed to form asperities on the surface of the catalyst layer 112 to be in contact with the polymer electrolyte membrane 111.